1. Field of the Invention
The present invention relates to a recovery boiler for recovering a soda component etc. from pulp spent liquor etc.
2. Description of the Prior Art
In a pulp spent liquor generated in a paper making process, there are contained, in a large quantity, a portion of organic matter of wood materials and a soda component added in the process of cooking. The spent liquor (hereinafter referred to as "black liquor") is once condensed and then burnt in a recovery boiler comprising a furnace. A main object is to recover the soda component so that the soda component in the black liquor is recovered as a sodium carbonate and a sodium sulfide in a molten state.
FIG. 8 is a schematic view showing a recovery boiler in the prior art. A black liquor 42a is ejected from a plurality of black liquor burners 42 into a recovery boiler 41. On the other hand, a combustion air 40 is fed thereinto from a primary air nozzle 45a, a secondary air nozzle 45b and a tertiary air nozzle 45c via a fan 43 and air dampers 44a, 44b and 44c. The black liquor 42a is burned on a char bed 46 formed at a lower portion of the boiler.
In the recovery boiler 41, the black liquor 42a, containing comparatively coarse particle sizes, is ejected from the black liquor burners 42 to a mid portion of the furnace, as shown by dotted line arrows in FIG. 8. The liquor 42a falls while it is being dried by a furnace combustion gas and is then accumulated on a furnace floor so as to form the char bed 46 to be burned.
With the enhancement of evaporator performance to condense the water content from spent liquor, it is a recent tendency that a solid concentration in the black liquor 42a, which had so far been 60%, has been increased to 80%. The result is that boiler combustion efficiency has been increased, and also the concentration of the black liquor itself is enhanced.
Also, a sufficient quantity of the primary air, the secondary air and the tertiary air is being fed to the surroundings of the char bed 46 and the degree of combustion of the black liquor 42a on the char bed 46 is enhanced, which results in a combustion state causing a sharp rise of nitrogen oxides (hereinafter referred to as "NO.sub.x "), an object of pollution control regulations. Thus in order to operate the recovery boiler in accordance with pollution control regulations, it is indispensable to reduce the quantity of NO.sub.x discharged from the furnace outlet.
One method for reducing the NO.sub.x considered is to generate a combustion zone of a reduction atmospheric field in which the air ratio in the surroundings of the char bed is 0.8 or less and to feed additional air from an upper portion of the furnace. In this case, however, a quantity of all or any of the primary air, the secondary air and the tertiary air is necessarily reduced, the flow velocity of air fed into the furnace is lowered, and the air quantity distribution in the furnace becomes irregular, so that there occurs a non-uniform combustion, a carry-over of unburnt char, a deformation of the char bed, etc. This makes the holding of stable combustion difficult.